Heretofore, thermoplastic elastomers have been widely used as materials for bumper parts, automotive interior trims such as instrument panels and interior sheets, or golf club grips or swimming fins. The thermoplastic elastomers referred to above have characteristics of both thermoplasticity and elasticity, and are capable of being formed by injection or extrusion molding technique or the like into molded articles which are excellent in heat resistance, tensile characteristics, weatherability, flexibility and elasticity.
For example, Japanese Patent Publn. No. 34210/1978 discloses thermoplastic elastomers prepared by partially curing a composition containing 60-80 parts by weight of monoolefin copolymer rubber and 40-20 parts by weight of polyolefin plastics under dynamic conditions. Japanese Patent Publn. No. 21021/1978 discloses thermoplastic elastomers which (a) a partially crosslinked copolymer rubber comprising an ethylene/propylene/nonconjugated polyene copolymer rubber and having a gel content of 30-90% by weight, and (b) a polyolefin resin. Further, Japanese Patent Publn. No. 18448/1980 discloses thermoplastic elastomers obtained by partially or completely crosslinking a composition containing ethylene/propylene copolymer rubber and a polyolefin resin under dynamic conditions.
Such thermoplastic elastomer compositions as cited above may be formed by an injection or extrusion molding technique or the like into molded articles excellent in heat resistance, tensile strength, weatherability, flexibility and elasticity. However, these thermoplastic elastomer compositions had such drawbacks that they are markedly poor in molding workability and flow characteristics, with the result that flow marks are formed on the surface of the resulting molded articles and the molded articles obtained deteriorate in external appearance.
With the view of solving such drawbacks as mentioned above, the present applicant disclosed by way of Japanese Patent Publn. No. 15741/1981 partially crosslinked thermoplastic elastomer compositions characterized in that said compositions are obtained by dynamically heat treating in the presence of organic peroxide a mixture comprising (a) 90-40 parts by weight of peroxide crosslinking type olefin copolymer rubber, (b) 10-60 parts by weight of peroxide decomposition type olefin plastics (herein (a) and (b) are selected so that (a)+(b) are 100 parts by weight), and 5-100 parts by weight of (c) peroxide noncrosslinking type hydrocarbon rubbery substance and/or (d) mineral oil type softener. By using the thermoplastic elastomer compositions prepared according to the process disclosed in this Japanese Patent Publn. No. 15741/1981, however, no molded articles having JIS A type spring hardness (JIS K-6301) of not more than 50 were obtained.
Recently, however, composite materials for use in automotive trim materials, which are prepared by a two-color injection molding technique using, for example, a filler-incorporated polypropylene as a core and a thermoplastic elastomer as a surface material, are required to meet such requirements that the filler-incorporated polypropylene is to impart mechanical strength to said composite materials and the surface material composed of the thermoplastic elastomer is to impart low hardness, excellent soft touch, feelingness and cushioning properties to said composite materials. Under such circumstances, there has come to be desired the advent of such thermoplastic elastomer compositions as capable of providing therefrom molded articles having JIS A spring hardness of not more than 50 and excellent in soft touch, feelingness and cushioning properties.
In the present invention, it is an object of the invention to provide processes for preparing thermoplastic elastomer compositions from which molded articles low in hardness and excellent in soft touch, feelingness and cushioning properties are obtained and also to provide such thermoplastic elastomer compositions.